Rotary drill bits are frequently used to drill oil and gas wells, geothermal wells and water wells. Rotary drill bits may be generally classified as rotary cone or roller cone drill bits and fixed cutter drilling equipment or drag bits. Fixed cutter drill bits or drag bits are often formed with a matrix bit body having cutting elements or inserts disposed at select locations of exterior portions of the matrix bit body. Fluid flow passageways are typically formed in the matrix bit body to allow communication of drilling fluids from associated surface drilling equipment through a drill string or drill pipe attached to the matrix bit body. Such fixed cutter drill bits or drag bits may be referred to as “matrix drill bits.”
Matrix drill bits are typically formed by placing loose matrix material (sometimes referred to as “matrix powder”) into a mold and infiltrating the matrix material with a binder such as a copper alloy. The mold may be formed by milling a block of material such as graphite to define a mold cavity with features that correspond generally with desired exterior features of the resulting matrix drill bit. Various features of the resulting matrix drill bit such as blades, cutter pockets, and/or fluid flow passageways may be provided by shaping the mold cavity and/or by positioning temporary displacement material within interior portions of the mold cavity. A preformed steel shank or bit blank may be placed within the mold cavity to provide reinforcement for the matrix bit body and to allow attachment of the resulting matrix drill bit with a drill string.
Matrix bits, and in particular diamond impregnated matrix bits, are typically used for drilling hard rock formations such as granite using a grinding-type action. However, matrix bits often experience problems when drilling in formations that include hard rock formations interspersed with layers or inclusions of soft rock such as soft shale or limestone. As matrix bits drill through such portions of soft rock the resulting cuttings often have a relatively sticky consistency and are not thoroughly removed by the interaction between the matrix bit and drilling fluid. Additionally, the grinding action of the matrix bit is often ineffective in relatively soft formations. As a result, after a matrix bit passes through a layer of soft formation and returns to drilling hard rock, material may remain in the indentations, grooves and cavities of the drill bit, often interfering with the grinding-type action of the drill bit in the hard rock formation. This material often significantly decreases the overall effectiveness of the drill bit and significantly limits the application of matrix bits.